There are a number of design criteria which have long been sought for segmental bone replacement implants. These include that the implant should (1) last the lifetime of the patient without losing function or initiating any adverse process response; (2) restore the normal function of the bone in which it is implanted; and (3) be producible on a commercial scale. To satisfy the foregoing criteria, not only should the implant support the imposed load, often of a fluctuating nature, but the interface between the implant and the bone should also withstand the load requirement.
A plastic cement such as polymethyl methacrylate is often used to affix an implant to bone as well as to improve the fit between the implant and the bone. Implants also have been provided with porous coatings which mate with the bone and invite bone ingrowth such that, after a period of time, the prosthesis becomes integrated into the bone structure. Typical of such coatings are the those disclosed in U.S. Pat. Nos. 3,855,638; 4,206,516; 4,156,943; and 4,612,160.
Ceramic coatings have also been used to good effect and often are particularly desirable because of the affinity between bone and ceramic materials such as alumina (Al2O3). Typical of such coatings are those disclosed in U.S. Pat. Nos. 4,145,764 and 4,483,678 to which are particularly concerned with dental implants, and U.S. Pat. Nos. 4,309,488 and 4,846,837, which more broadly disclose implantable bone replacement material for use throughout the body.
An important method of promoting implant stability is osseointegration, i.e., forming a direct structural connection between living bone and the implant surface. It is generally known in the art that the osseointegration of metallic orthopaedic implants is dependent, in part, on the attachment and spreading of osteoblast-like cells on the surface of the orthopaedic implant. Studies suggest that such cells will more readily attach to rough or porous surfaces, as compared to smooth surfaces. As such, several attempts have been made to provide metallic orthopaedic implants having roughened and/or porous surfaces to aid in the osseointegration.
U.S. Pat. No. 5,236,459, for example, describes a process for forming an implant surface having “anchoring areas” in which a high-pressure liquid jet is used to remove a portion of the metal from the implant surface. The diameter of the “anchoring areas” can be varied from 0.5 to 1.5 mm.
U.S. Pat. No. 5,307,594 describes another method for forming a textured surface on orthopaedic implants. The method entails the application of a resilient mask, which contains several openings, to the surface of the implant and then subjecting the implant to high pressure blasting using a blasting media such as metal oxides particles. While this process can be used to produce implant surfaces having roughened surfaces, particles of the blasting media can become embedded in the surface of the implant. It is believed that these particles can negatively impact the osseointegration of the orthopaedic implant following implantation.
The methods described by the patents in the preceding paragraphs provide metallic implants having a roughened surface with surface features that are generally greater than 20 μm in size. While an orthopaedic implant having such surface features may exhibit improved osseointegration when compared to a smooth metallic implant, it is believed that osseointegration will be greatly improved if the implant surface includes smaller surface features (i.e., less than 20 μm in size).
Other work has utilized highly convoluted surfaces on the implant. U.S. Pat. Nos. 5,368,881 and 5,658,333 show use of non-spherical powder to produce a roughened surface for prosthesis. These surfaces, however, are known to have little to no inter-connected porosity.
In addition to the mechanical methods of providing a roughened surface described above, various chemical etching methods have been used to texture the surface of orthopaedic implants. U.S. Pat. No. 5,876,453, for example, describes a two-step process in which a hydrofluoric acid solution (10-50% HF) is used to remove the native oxide surface layer formed on the metallic implant, and a second acid treatment is used to further etch the metal to provide a roughened surface. The second acid treatment utilizes a mixture of two parts sulfuric acid (96% by weight H2SO4) and one part hydrochloric acid (37% by weight HCl). While this process and similar chemical etching processes are capable of producing roughened metallic implants having surface features at micron level, the process employs two very aggressive and highly concentrated acid solutions. One accordingly assumes a higher safety risk in storage and application than if less aggressive or low concentration acids are used.
Published U.S. Patent Application No. 2004/0167633 utilizes an etching solution comprising (i) at least one fluoride salt, (ii) at least one acid, and (iii) water for a time and under conditions sufficient to provide the implant with micron or nanometer-scale surface roughness.
Despite the progress made by the aforementioned art, there is a continued need for prosthesis surfaces with improved properties. In addition, there is a desire to avoid the use of high concentration acids such as HF or fluoride salts.